The politicians affected have teamed up with Sharon Grant, who is acting on behalf of her recently deceased politician husband Bernie Grant – who was among those placed under surveillance. Solicitors will present evidence condemning Westminster’s calls for police actions to remain secret.

In particular, it will be argued that withholding information on the targeting of democratically elected representatives hinders the inquiry’s capacity to hear vital evidence.

Officers, meanwhile, have drawn on human rights law to argue their case, with the backing of the Home Office. But McDonnell says any torment caused by undercover policing will not disappear until police chiefs unveil “the full truth.”

“It would be easier for the Metropolitan Police commissioner now to [be] fully open to the public and admit what went on, give us all the information that we need,” he told the Morning Star on Tuesday.

“And in that way, maybe, maybe, some of the victims can get some form of closure on this — they won’t, otherwise.”

But Scotland Yard has called on the inquiry chairman to maintain the force’s “neither confirm nor deny” policy over the identity of undercover police and details of their work in the field.

UK campaign Police Spies Out of Lives has previously said the victims of police surveillance strongly support the “principle of open justice.”